You can buy a commercial and purpose-made weight for making your own bob. You can also make your own weight. Use anything heavy but it should in a round shape and should not be too large in size. You can use a bunch of old keys that locksmiths give away because they are stuck incorrectly. The selection of weight also depends on the item you want to straighten up.
Selection of cord is also a careful process. Consider the weight you have chosen while selecting a cord. For something lighter, a nylon cord will do but if the weight is too heavy, select a steel cable or cord. Make sure the cord is durable, as a weak cord can be broken and cause a safety issue, especially if you have children at your house.
Tie Cord To Weight
After taking the cord at the length you want, tie its one end to the weight. Make sure you secure the end with the weight properly. You may use some sort of clip to tie the cord tightly with the weight before leave it loose. This part needs special care as if the cord is not tied with the weight properly, it can make the weight fall and can cause a safety risk.
Take a S-shaped hook and tie it on other end of the cord. This will reduce the weight on the cord. Also, it will make it easier for you to take the plumb bob to different places in the house for different uses. While using bob, you can tie it round with a small wire or rope, so it does not swing back and forth. It can damage cord or cause a safety risk. Make sure you keep plumb bob at a safe place after its use.